Fluorocarbon based fluids have found widespread use in industry for refrigeration, air conditioning and heat pump application.
Vapor compression is one form of refrigeration. In its simplest form, the vapor compression involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes the refrigerant to vaporize. The vapor formed is then removed by means of a compressor in order to maintain the low pressure in the evaporator. The temperature and pressure of the vapor are then raised through the addition of mechanical energy by the compressor. The high pressure vapor then passes to the condenser whereupon heat exchanges with a cooler medium. The sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.
While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.
Certain chlorofluorocarbons have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures. The use of azeotropic mixtures as refrigerants is known in the art. See for example, R. C. Downing, "Fluorocarbon Refrigerants Handbook", pp. 139-158, Prentice-Hall, 1988, and U.S. Pat. Nos. 2,101,993 and 2,641,579.
Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling or evaporation. This behavior is desirable because in the previously described vapor compression equipment with which these refrigerants are employed, condensed material is generated in preparation for cooling or for heating purposes and unless the refrigerant composition is constant boiling, fractionation and segregation will occur upon evaporation and condensation and undesirable refrigerant distribution may act to upset the cooling or heating.
Non-azeotropic mixtures have been disclosed as refrigerants, see e.g., U.S. Pat. No. 4,303,536, but have not found widespread use in commercial applications. Because nonazeotropic mixtures may fractionate during the refrigeration cycle, certain hardware changes must be made when they are used. It is primarily because of this added difficulty in changing and servicing refrigeration equipment that non-azeotropic refrigerants have been avoided. The situation is further complicated if an inadvertent leak in the system occurs during use or servicing. The composition of the mixture could change, affecting system pressures and system performance. If one component of the non-azeotropic mixture is flammable, then fractionation could shift the composition into the flammable region with potentially adverse consequences.
Trichlorofluoromethane (FC-11) has been routinely used as a refrigerant in large capacity water chillers, which are used to provide air conditioning for large buildings and industrial applications. Because dichlorotrifluoroethane (FC-123 or FC-123a) and 1,2-difluoroethane (FC-152) have boiling points greater than FC-11, they have vapor pressures less than the vapor pressure of FC-11 at the same temperature. As a result, their refrigeration capacity is less than that of FC-11. The azeotropic mixture of FC-123 and FC-152 exhibits a minimum boiling point, that is, it is more volatile than either FC-123 or FC-152 and thus possesses a greater refrigeration capacity, which more closely matches that of FC-11. Furthermore, FC-152 is flammable while FC-123 and FC-11 are nonflammable. The azeotrope-like mixtures of FC-123 and FC-152 are less flammable than FC-152 and do not segregate or fractionate upon evaporation or condensation.
Rigid polyurethane and polyisocyanurate foams are manufactured by reacting and foaming a mixture of ingredients comprising, in general, an organic isocyanate such as pure or crude toluene diisocyanate or a polymeric diisocyanate, with an appropriate amount of polyol, or mixture of polyols, in the presence of a volatile liquid blowing agent, which vaporizes during the reaction, causing the polymerizing mixture to foam. The reactivity of these ingredients is enhanced through the use of amine and/or tin catalysts and surfactant materials which serve to control and adjust cell size as well as to stabilize the foam structure during its formation.
In the production of flexible polyurethane foams water and excess diisocyanate are employed. The diisocyanate reacts with the water producing gaseous carbon dioxide which, in turn, causes foam expansion. Flexible foams are widely used as cushioning materials in items such as furniture, bedding and automobiles. Auxiliary physical blowing agents such as methylene chloride and/or trichlorofluoromethane are required in addition to the water/diisocyanate blowing mechanism in order to produce low density, soft grades of flexible polyurethane foam.
Rigid polyurethane and polyisocyanurate foams are almost exclusively expanded using trichlorofluoromethane (FC-11) as the blowing agent. Some rigid foam formulations do incorporate small amounts of water in addition to the FC-11, but the FC-11 is the major blowing agent component. Other formulations sometimes use small amounts of the more volatile dichlorodifluoromethane (FC-12) in addition to FC-11 for producing so-called froth-type foams. Rigid foams are closed-cell foams in which the FC-11 vapor is trapped in the matrix of cells. These foams offer excellent thermal insulation characteristics, due in part to the low vapor thermal conductivity of FC-11, and are used widely in thermal insulation applications such as roofing systems, building panels, refrigerators, freezers and the like.
Three important requirements for a rigid polyurethane or polyisocyanurate foam blowing agent are expansion efficiency, i.e., the gas volume generated per unit weight blowing agent; the vapor thermal conductivity of the blowing agent, and the flammability of the blowing agent. For economic reasons, a highly efficient expansion agent is preferred. A blowing agent with a low vapor thermal conductivity is also preferred as the rigid foams are often employed as thermal insulation materials and the blowing agent thermal conductivity is an important contribution to the overall foam thermal conductivity. A nonflammable blowing agent is preferred for safety reasons.
Because FC-152 has a low molecular weight, it might be considered a good blowing agent from an expansion efficiency view point, i.e., less mass of FC-152 would be required to expand the foam to the same density, compared, for example, with either FC-11 or FC-123. However, a disadvantage of using FC-152 as a blowing agent is that it is flammable and is expected to have a high vapor thermal conductivity because of its low molecular weight, both of which detract from its performance as a blowing agent.
FC-123 might be considered a good blowing agent because it is nonflammable. However, a disadvantage of FC-123 as a blowing agent is that FC-123 has a high molecular weight and as a result, FC-123 is not an efficient blowing agent. The azeotrope-like blends of FC-123 and FC-152 possess a lower molecular weight than FC-123 alone. Therefore, they are more efficient blowing agents than FC-123 alone. The azeotrope-like blends of 123/152 are also less flammable than FC-152 alone. Furthermore, the azeotrope-like blends do not fractionate or segregate upon boiling or evaporation.
The azeotropic FC-123/FC-152 mixtures, depending on the FC-152 composition, are either nonflammable or are significantly less flammable than FC-152, have improved expansion efficiency compared to FC-123 and FC-11, and have a lower thermal conductivity than FC-152. Because the mixture is an azeotrope, it will not segregate into components upon evaporation, leading to a potentially flammable situation.
Recently, non-toxic, non-flammable fluorocarbon solvents, like trichlorotrifluoroethane, have been used extensively in degreasing applications and other solvent cleaning applications. Trichlorotrifluoroethane has been found to have satisfactory solvent power for greases, oils, waxes and the like. It has therefore found widespread use for cleaning electric motors, compressors, heavy metal parts, delicate precision metal parts, printed circuit boards, gyroscopes, guidance systems, aerospace and missile hardware, aluminum parts and the like.
The solvent art has looked towards azeotropic compositions having fluorocarbon components because the fluorocarbon components contribute additional desired characteristics, such as polar functionality, increased solvency power, and stabilizers. Azeotropic compositions are desired because they do not fractionate upon boiling. This behavior is desirable because in the previously described vapor degreasing equipment with which these solvents are employed, redistilled material is generated for final rinse-cleaning. Thus, the vapor degreasing system acts as a still. Therefore, unless the solvent composition is essentially constant boiling, fractionation will occur and undesirable solvent distribution may act to upset the cleaning and safety of processing. For example, preferential evaporation of the more volatile components of the solvent mixtures, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency towards soils, less inertness towards metal, plastic or elastomer components, and increased flammability and toxicity.
The art is continually seeking new fluorocarbon based azeotrope-like mixtures which offer alternatives for refrigeration and heat pump applications; blowing agents; and solvents for vapor degreasing and cold cleaning applications. Currently, environmentally acceptable fluorocarbon-based azeotrope-like mixtures are of particular interest, because the presently used fully halogenated chlorofluorocarbons have been implicated in causing environmental problems associated with the depletion of the earth's protective ozone layer.
Mathematical models have substantiated that partially halogenated species, like dichlorotrifluoroethane and 1,2-difluoroethane will not adversely affect atmospheric chemistry, since they contribute negligibly to stratospheric ozone depletion and global warming in comparison to the fully halogenated species. Atmospheric models have shown that FC-123 possesses an ozone depletion potential and global warming potential more than 50 times lower than that of FC-11. FC-152 does not contain chlorine and thus has zero potential for stratospheric ozone depletion. The azeotrope-like mixtures of FC-123 and FC-152 therefore possess improved environmental characteristics over FC-123 alone.
R. C. Downing, in "Fluorocarbon Refrigerants Handbook", p. 140 Prentice-Hall, (1988), discloses an azeotropic mixture of FC-11 and 1,1-dichloro-2,2,2-trifluoroethane (FC-123) as a refrigerant. U.S. Pat. No. 3,940,342 discloses azeotropic mixtures of FC-11 and 1,2-dichloro-1,1,2-trifluoroethane (FC-123a).
Commonly assigned U.S. Pat. No. 4,624,970, discloses mixtures of FC-11 and FC-123 or FC-123a, to expand polyurethane-type foams. Application U.S. Ser. No.: 240,655 teaches the use of azeotrope-like mixtures comprising FC-11, FC-123 (or FC-123a) and isopentane as blowing agents for polyurethane foams.
U.S. Pat. No. 4,816,176 discloses azeotrope-like compositions of dichlorotrifluoroethane, methanol and nitromethane. U.S. Pat. No. 4,816,175 teaches the use of azeotrope-like compositions of dichlorotrifluoroethane, methanol, nitromethane, and cyclopentane as solvents. These teachings do not suggest the present azeotropic composition because, as is known in the art, no published method exists for predicting the formation of an azeotrope.
It is accordingly an object of this invention to provide novel azeotrope-like compositions based on dichlorotrifluoroethane and 1,2-difluoroethane which are useful in cooling and heating applications, foam blowing applications, and solvent cleaning applications.
Another object of the invention is to provide environmentally acceptable azeotrope-like compositions for use in the aforementioned applications.
Other objects and advantages of the invention will become apparent from the following description.